Up-conversion references that are the work of at least one of the co-inventors includes U.S. Pat. No. 5,089,860 issued to Deppe et al. on Feb. 18, 1992. This patent describes a quantum well device with control of spontaneous photon emission and method of manufacturing, wherein spontaneous photon emission intensity in a semiconductor quantum well is strongly influenced by a highly reflecting interface with the quantum well interface spacing being less than the optical emission wavelength of the quantum well.
Other patents describing the inventor's work in the field of up-conversion includes U.S. Pat. Nos. 6,327,074 and 6,501,590 issued to Bass et al. respectively on Dec. 4, 2001 and Dec. 31, 2002, which are assigned to the same assignee as the subject invention. The Bass patents describe display mediums using emitting particles that are dispersed in a transparent host. The two and three dimensional color image displays include a display medium having a substantially uniform dispersion of red, green and blue visible light emitting particles sized between approximately 0.5 to approximately 50 microns therethrough. The particles can be dye doped polymethylmethacrylate (pmma) plastic, and the display medium can be pmma, acrylic plastic or glass. Other particles can be used such as rare earth doped crystals. The two dimensional display uses three laser sources each having different wavelengths that direct light beams to each of three different types of particle in the display medium. Light is absorbed by the particles which then become excited and emit visible fluorescence. Modulators, scanners and lens can be used to move and focus the laser beams to different pixels in order to form the two dimensional images having different visible colors.
Another patent describing the inventor's work in the field of up-conversion includes U.S. Pat. No. 6,654,161 issued to Bass et al. on Nov. 25, 2003, which is also assigned to the same assignee as the subject invention describes dispersed crystallite up-conversion displays based on up conversion of near infrared light to visible light. The display medium is a transparent polymer containing particles of crystals doped with Yb.sup.3+ and other rare earth ions. The Yb.sup.3+ ions absorb light from a commercially available diode laser emitting near 975 nm and transfers that energy to the other dopant ions. Using a fluoride crystal host, NaYF.sub.4, co-doped with Tm.sup.3+ ions blue light at about 480 nm was obtained, with Ho.sup.3+ or Er.sup.3+ ions green light at about 550 nm is obtained and with Er.sup.3+ red light at about 660 nm is obtained. The display medium can be used with applications for full color, high brightness, high resolution, displays.
U.S. Pat. No. 6,844,387 issued to Bass et al. on Jan. 18, 2005 is another patent describing the inventor's own work, which is also assigned to the same assignee as the subject invention describes composites of inorganic luminophores stabilized in polymer hosts. The two and three dimensional display medium can have a novel transparent polymer composite containing particles of crystals doped with Yb.sup.3+ and other rare earth ions. The polymer composite creates homogeneously dispersed compositions without cracking or delamination of the film and can be used for various optical applications.
U.S. Pat. No. 6,897,999 issued to Bass et al. on May 24, 2005 is another patent describing the inventor's own work and having the same assignee as that of the subject invention discloses an optically written display. The two, three dimensional color displays can include uniform dispersion of red, green and blue visible light emitting micron particles. Pumping at approximately 976 nm can generate green and red colors having an approximately 4% limit efficiency. One light source can generate three colors with a low limit efficiency. Modulators, scanners and lens can move and focus laser beams to different pixels forming two dimensional color images. Displays can be formed from near infrared source beams that are simultaneously split and modulated with micro electro mechanical systems, spatial light modulators, liquid crystal displays, digital micro mirrors, digital light projectors, grating light valves, liquid crystal silicon devices, polysilicon LCDs, electron beam written SLMs, and electrically switchable Bragg gratings. Pixels containing Yb,Tm:YLF can emit blue light. Pixels containing Yb,Er(NYF) can emit green light, and pixels containing Yb,Er:KYF and Yb,Ef:YF.sub.3 can emit red light.
The concept of frequency up-conversion (UC) of infrared-to-visible light in rare-earth (RE) doped materials was reported more than forty years ago for the first time. The efficiency that was observed or expected for this process was low in singly doped media. It was quickly noticed that up-conversion could be made one or two orders of magnitude more efficient by using ytterbium (Yb) as a sensitizer ion in addition to the active ion: erbium (Er), holmium (Ho), or thulium (Tm).
In years past, efficient up-conversion materials were investigated, for photonic applications, such as in UC lasers (visible lasers that are pumped by infrared diode lasers), or in display applications. However, because no powerful source existed in the 980-nm region in order to excite those up-converters, no practical product came out of the research. With the development of powerful 980-nm diode lasers lead by the telecommunication industry, there can now be legitimate practical applications.
The prior art on the subject included pumping conditions that caused heating of the material and higher efficiencies were obtained with low duty cycle excitation. It was also reported that for the same average input power, higher efficiencies were expected in pulsed excitation mode rather than in continuous wave excitation due to the quadratic nature of the process.
The effect of the pumping conditions for display applications of up conversion materials needs to be understood, as several technologies might be used to form the image. The infrared source can either be scanned (vector-addressed or raster-scan), or the image can be directly projected using Digital Micromirror Devices (MEMS) such as in the Texas Instrument Digital Light Processing (DLP.TM.) technology. In the latter case the materials would be undergoing pulse-excitation, whereas they would be quasi-continuously excited in the second case.
U.S. Pat. No. 7,075,707 issued to Rapaport et al. on Jul. 11, 2006, to the same assignee as that of the subject invention, describes a substrate design for optimized performance of up-conversion phosphors utilizing proper thermal management. The patent describes methods and compositions for using an up-conversion phosphor as an emitting material in a reflective display and polymer compositions for display mediums, and red, green, blue (RGB) display mediums. Roles of the pumping duration and character on the temperature and the efficiency of the up-conversion process in (Ytterbium, Erbium or Thulium) co-doped fluoride crystals are also described. A problem with prior art up-conversion devices is limited efficiency since much of the incident pump light is back scattered by the up-converting particles and does not get used to generate visible light.
“Review of the properties of Up-Conversion Phosphors for new Emissive Displays” Alexandra Rapaport, Janet Milliez, Michael Bass, Arlete Cassanho, and Hans Jenssen, Invited Review article in IEEE J. of Display Technology 2, pp. 68-78, March 2006
U.S. Pat. No. 7,471,706 titled “High resolution, full color, high brightness fully integrated light emitting devices and displays” issued on Dec. 30, 2008 to co-inventors Michael Bass and Dennis G. Deppe” describes a light emitting device that includes a substrate, at least one semiconductor light emitting device formed in or on the substrate, and upconverting material disposed in or on the substrate. The upconverting material is disposed in a path of light processed or emitted by the semiconductor device. The upconverting material absorbs light emitted by the semiconductor device and emits upconverted light in response. Integrated pixelated displays include a plurality of pixels formed on a surface of the substrate, with each pixel including up-conversion material based red light source, a blue light source a green light source, and a structure for selectively controlling emission from the red, blue and green lights sources for each of the pixels.
Recently filed U.S. patent application Ser. No. 12/349,712 filed on Jan. 7, 2009 describing the work of the co-inventors describes methods and systems for a combination of up converters and semiconductor light sources in low voltage display or indicator system. The display or indicator system includes one or more spatial light modulators and one or more up converters in combination with one or more semiconductor light sources.
U.S. patent application Ser. No. 12/124,620 filed May 22, 2008 titled “Composite cavity for enhanced efficiency of up conversion” and Ser. No. 12/124,234 filed on May 22, 2008 titled “Combination of up converting materials with oxide confined semiconductor light sources” , also work of the co-inventors, describe methods, apparatus and systems for an up-converter resonant cavity light emitting diode device includes a semiconductor light source, an up-converter to form the light emitter with up-converting materials and an electrical source coupled with the semiconductor light source for providing electrical energy to the semiconductor light source to provide a desired wavelength emitted light. The semiconductor light source is a resonant cavity light emitting diode or laser that emits an approximately 975 nm wavelength to provide electrical and optical confinement to the semiconductor light source to form a resonant cavity up-converting light emitting diode.
U.S. patent application Ser. No. 12/365,971 filed by the co-inventors on Feb. 5, 2008 describes light sources using up converters to generate visible light in which the infrared light excitation is confined in a waveguide like structure.
Prior art employs visible LEDs directed into fibers in which scatterers have been placed to divert some of the light towards the viewer. Only one color can be generated and only where the scattering item is located. The range of colors available is limited to that available to visible LEDs and the shorter wavelengths (blue, green and yellow) require operation at 4 V necessitating expensive, large and heavy battery packs.